You can use ArcgIS Online Assistant to do that. First, make a copy of the dashboard (in case it gets messed up). From the "I want to" dropdown, select "View an Item's JSON". Select your dashboard copy and in the Data portion of the code, click the Edit icon. In the widget you want to change, look for the "dataSource" property and change the "itemId" property. Click the Save icon and view the dashboard to see if it worked properly.
